Method for liquefying a hydrocarbon-rich fraction

ABSTRACT

A method for liquefying a hydrocarbon-rich fraction, in particular natural gas, by indirect heat exchange with the refrigerant blend of a refrigerant blend circuit is described, wherein the refrigerant blend is compressed, separated into a liquid phase which is rich in higher-boiling components (HMR) of the refrigerant blend and a gas phase which is rich in lower-boiling components (LMR) of the refrigerant blend, and said phases are mixed before the indirect heat exchange. According to the invention, indirect heat exchange proceeds in at least two heat exchangers (E 3,  E 4 ), wherein the first heat exchanger (E 4 ) serves to precool and the second heat exchanger (E 3 ) to liquefy the hydrocarbon-rich fraction, and the first heat exchanger is supplied with a refrigerant blend which comprises 5 to 50% of the liquid phase ( 3, 15 ) which is rich in higher-boiling components (HMR) of the refrigerant blend and said blend is combined with the gas phase ( 6, 14 ) which is rich in lower-boiling components (LMR) of the refrigerant blend in such a way that an HMR/LMR mixing ratio of between 1.2 and 10 is established.

The invention relates to a method for liquefying a hydrocarbon-rich fraction, in particular natural gas, by indirect heat exchange with the refrigerant blend of a refrigerant blend circuit, wherein the refrigerant blend is compressed, separated into a liquid phase which is rich in higher-boiling components (HMR=Heavy Mixed Refrigerant) of the refrigerant blend, and a gas phase which is rich in lower-boiling components (LMR=Light Mixed Refrigerant) of the refrigerant blend, and these phases are mixed before the indirect heat exchange.

Methods for liquefying hydrocarbon-rich fractions or gas mixtures, in particular natural gas, make use, inter alia, of closed refrigerant blend circuits in which the multicomponent refrigerant is at least partially condensed under elevated pressure at around ambient temperature and is vaporized at low pressure at below ambient temperature with a refrigerating action. In simple methods, just one refrigerant blend circuit is used, in which the refrigerant fractions arising during compression are mixed before the indirect heat exchange with the hydrocarbon-rich fraction to be liquefied and jointly used in the heat exchanger.

With reference to the procedure shown in FIG. 1, a method of this generic type for cooling and liquefying a hydrocarbon-rich fraction, as is for example disclosed in German patent application 10 2011 010 633 (US 2012/0198883), is explained in greater detail below.

The hydrocarbon-rich fraction to be cooled and liquefied, which is for example natural gas, is supplied via line 100 to the heat exchanger E3′. In the latter, the feed fraction is cooled against the refrigerant blend circuit which is yet to be described and supplied via line 101 to a separation unit T. This separation unit T, which is simply shown as a black box, serves for example to separate nitrogen and/or higher hydrocarbons from the feed fraction 100/101 to be liquefied. The separation process performed in the separation unit T determines the temperature to which the feed fraction 100/101 must at least be cooled in the heat exchanger E3′. The component(s) separated from the feed fraction is/are drawn off from the separation unit T via line 104, while the remaining feed fraction to be liquefied is supplied again via line 102 to the heat exchanger E3′ and is further cooled, liquefied and optionally supercooled therein. The feed fraction 103 treated in this manner is then sent for further use or to a storage tank.

The refrigerant blend circuit required for cooling and liquefying the hydrocarbon-rich feed fraction 100/102 comprises an at least two-stage compressor unit C, a separator D1 upstream of the compressor unit C and two separators D2 and D3 downstream of the compressor stages. Two post-coolers E1 and E2, which serve to dissipate the heat of compression and partially condense the refrigerant blend, and a pump or pump unit P are furthermore provided.

The refrigerant blend vaporized in the heat exchanger E3′ against the feed fraction 100/102 to be cooled and liquefied is supplied via line 1 to the above-stated separator D1. The gas phase drawn off from the top of this separator via line 1′ is supplied to the first compressor stage of the compressor unit C and compressed to a desired intermediate pressure. After passing through the post-cooler E1, the compressed refrigerant blend is supplied via line 2 to the separator D2. A liquid phase which is rich in higher-boiling components of the refrigerant (HMR) is drawn off via line 3 from the bottom of the separator and pumped by means of the pump or pump unit P to the pressure of the gas phase which is yet to be described of the refrigerant blend.

The gas phase drawn off via line 4 from the separator D2 is supplied to the second stage of the compressor C and compressed to the desired final pressure of the refrigerant blend circuit. After passing through the post-cooler E2, the compressed refrigerant blend is supplied via line 5 to the separator D3. The liquid fraction 7 arising in the bottom of the separator D3 is recirculated via the control valve V1 before the input of the separator D2. A gas phase which is rich in lower-boiling components of the refrigerant blend (LMR) is drawn off at the top of the separator D3 via line 6 and, after mixing with the above-described liquid phase 3, is supplied via line 8 to the heat exchanger E3′. The liquid phase 3 and the gas phase 6 are combined before the heat exchanger or immediately at the start of the heat exchange which proceeds in the heat exchanger E3′ and supplied as a two-phase stream. The refrigerant blend is cooled in the heat exchanger E3′ and completely liquefied. At the cold end of the heat exchanger E3′, the refrigerant blend 9 is expanded with a refrigerating action in the valve V2 and then completely vaporized on passing again through the heat exchanger E3′.

Using the above-described procedure, however, it is not possible purposefully to influence the temperature profile in the heat exchanger E3′. The available, fluctuating variables of the refrigerant blend circuit, such as pressure profile, mass flow rate and composition, are used to control system capacity and the temperature of the feed fraction at the cold end of the heat exchanger E3′ and to optimize energy consumption. If a desired intermediate temperature in the heat exchanger E3′ is now required in the course of gas liquefaction, for example to avoid precipitation of solids in the feed gas or to establish a desired separation of substances, such as for instance the above-described separation of nitrogen or higher hydrocarbons, the intermediate temperature is not controllable independently of the load and temperature of the fraction to be liquefied at the cold end of the heat exchanger E3′.

An object of the present invention is to provide a method for liquefying a hydrocarbon-rich fraction, in particular natural gas, which makes it possible to achieve sufficiently accurate control of a further temperature in addition to the temperature at the cold end of the heat exchanger used for indirect heat exchange. This should be taken to mean control to at least 3° C., preferably to at least 1° C.

Upon further study of the specification and appended claims, other objects, aspects and advantages of the invention will become apparent.

These objects are achieved by providing a method of this generic type for liquefying a hydrocarbon-rich fraction, in particular natural gas, which is characterized in that

-   -   indirect heat exchange proceeds in at least two heat exchangers,     -   wherein the first heat exchanger serves to precool and the         second heat exchanger to liquefy the hydrocarbon-rich fraction,         and     -   the first heat exchanger is supplied with a refrigerant blend         which comprises 5 to 50% of the liquid phase which is rich in         higher-boiling components (HMR) of the refrigerant blend and the         blend is mixed with the gas phase which is rich in lower-boiling         components (LMR) of the refrigerant blend in such a way that an         HMR/LMR mixing ratio of between 1.2 and 10 is established.

In the above-described, prior art method for liquefying a hydrocarbon-rich fraction, the liquid phase and the gas phase of the refrigerant blend are in each case mixed in their entirety and jointly used for cooling and liquefying the feed fraction. According to the invention, indirect heat exchange between the hydrocarbon-rich fraction and the refrigerant blend now proceeds in at least two heat exchangers, wherein the first heat exchanger serves to precool and the second heat exchanger to cool and liquefy the hydrocarbon-rich fraction. The first or precooling heat exchanger is here predominantly cooled with the liquid phase of the refrigerant blend, while the second heat exchanger or liquefier is predominantly cooled with the gas phase of the refrigerant blend. According to the invention, the first heat exchanger is therefore supplied with a refrigerant blend which comprises 5 to 50% of the liquid phase which is rich in higher-boiling components (HMR) of the refrigerant blend. This liquid phase is mixed with the gas phase which is rich in lower-boiling components (LMR) of the refrigerant blend in such a way that an HMR/LMR mixing ratio of between 1.2 and 10 is established. The remaining proportions of the liquid phase and gas phase are used to cool the second heat exchanger. The refrigerant blend used for the first heat exchanger is now concentrated by a multiple in higher-boiling components and is accordingly higher-boiling. The refrigerant blend of the second heat exchanger is consequently concentrated in lower-boiling components of the refrigerant blend and accordingly lower-boiling.

The refrigeration capacities and temperature profiles of the two heat exchangers may now be influenced via the mixtures and quantities of the respective refrigerant fractions in such a way that the temperature at the cold end of the first heat exchanger, and likewise the temperature at the cold end of the second heat exchanger, can be accurately controlled to at least 3° C., preferably to at least 1° C.

Further advantageous developments of the method according to the invention for liquefying a hydrocarbon-rich fraction are characterized in that

-   -   the first heat exchanger is supplied with a refrigerant blend         which comprises 10 to 30% of the liquid phase which is rich in         higher-boiling components (HMR) of the refrigerant blend,     -   an HMR/LMR mixing ratio of between 2 and 5 established, and/or     -   a sub-stream of the gas phase is supplied to the refrigerant         blend at the cold end of the first and/or the second heat         exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention for liquefying a hydrocarbon-rich fraction and further advantageous developments thereof will be explained in greater detail below with respect to the figures wherein:

FIG. 1 shows a prior art system for cooling and liquefying a hydrocarbon-rich fraction (discussed in detail above); and

FIG. 2 depicts an exemplary embodiment of the invention.

The hydrocarbon-rich fraction 200 to be cooled and liquefied is now supplied to a first heat exchanger or precooler E4. In the latter, the feed fraction is cooled against the refrigerant blend circuit which is yet to be described and supplied via line 201 to a separation unit T. The component(s) separated from the feed fraction is/are drawn off from the separation unit T via line 204, while the remaining feed fraction to be liquefied is supplied again via line 202 to the second heat exchanger or liquefier E3 and is further cooled, liquefied and optionally supercooled therein. The feed fraction 203 treated in this manner is then sent for further use or to a storage tank.

With the exception of the distribution of the gas phase 6 and liquid phase 3 between the two heat exchangers E3 and E4, the refrigerant blend circuit required for cooling and liquefying the hydrocarbon-rich feed fraction 200/202 corresponds to the refrigerant blend circuit explained with reference to FIG. 1. Therefore only the differences relative to the refrigerant blend circuit explained with reference to FIG. 1 will now be addressed below.

According to the invention, the liquid phase 3 drawn off from the bottom of the separator D2 is distributed by means of the control valves V6 and V7 via the line portions 11 and 15 between the heat exchangers E3 and E4. The heat exchanger E4 is here supplied with a refrigerant blend which comprises 5 to 50%, preferably 10 to 30%, of the liquid phase which is rich in higher-boiling components (HMR) of the refrigerant blend. The distribution of the gas phase 6 which is drawn off at top of the separator D3 and is rich in lower-boiling components (LMR) of the refrigerant blend via the line portions 10 and 14 between the heat exchangers E3 and E4 is determined by the mass balance of the combined refrigerant blend streams 12 and 16 via the valves V2 and V4.

Sub-streams of the gas phase 6 may be supplied via the line portions 13 and 17 to the refrigerant blend 12 or 16 respectively at the cold end of the first and/or the second heat exchanger E4 or E3 respectively. The control valves V3 and V5 provide a further possibility for temperature control at the cold end of the heat exchangers E3 and E4. In addition, it is possible by means of the two valves V3 and V5 to establish a minimum gas velocity which ensures stable cold running of the heat exchangers E3 and E4 by preventing segregation of the gas phase and liquid phase during vaporisation.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German patent application 10 2013 016 695.0, filed Oct. 8, 2013, are incorporated by reference herein. 

1. A method for liquefying a hydrocarbon-rich fraction by indirect heat exchange with a refrigerant blend of a refrigerant blend circuit, said method comprising: performing said indirect heat exchange between said hydrocarbon-rich fraction and said refrigerant blend in at least two heat exchangers (E3, E4) comprising a first heat exchanger (E3) and a second heat exchanger (E3), wherein said first heat exchanger (E4) serves to precool said hydrocarbon-rich fraction, and said second heat exchanger (E3) serves to liquefy said hydrocarbon-rich fraction (200-203), wherein said refrigerant blend is compressed, separated into a liquid phase, which is rich in higher-boiling components (HMR) of the refrigerant blend, and a gas phase, which is rich in lower-boiling components (LMR) of the refrigerant blend, and said liquid phase and said gas phase are mixed before said indirect heat exchange, and wherein said first heat exchanger (E4) is supplied with a first portion of said refrigerant blend and said second heat exchanger (E4) is supplied with a second portion of said refrigerant blend, wherein said first portion of said refrigerant blend comprises 5 to 50% of said liquid phase (3, 15) rich in higher-boiling components and an amount of said gas phase (6, 14) rich in lower-boiling components (LMR) in such a way that an HMR/LMR mixing ratio of between 1.2 and 10 is established in said portion of the said refrigerant blend.
 2. The method according to claim 1, wherein said portion of said refrigerant blend supplied to said first heat exchanger (E4) comprises 10 to 30% of said liquid phase rich in higher-boiling components.
 3. The method according to claim 1, wherein the HMR/LMR mixing ratio of said portion of said refrigerant blend is between 2 and 5 is established.
 4. The method according to claim 2, wherein the HMR/LMR mixing ratio of said portion of said refrigerant blend is between 2 and 5 is established.
 5. A method according to claim 1, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the first heat exchanger (E4).
 6. A method according to claim 2, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the first heat exchanger (E4).
 7. A method according to claim 3, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the first heat exchanger (E4).
 8. A method according to claim 4, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the first heat exchanger (E4).
 9. A method according to claim 1, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 10. A method according to claim 2, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 11. A method according to claim 3, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 12. A method according to claim 4, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 13. A method according to claim 5, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 14. A method according to claim 6, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 15. A method according to claim 7, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 16. A method according to claim 8, wherein a sub-stream (13, 17) of said gas phase (6) is supplied to the refrigerant blend (12, 16) at the cold end of the second heat exchanger (E3).
 17. A method according to claim 1, wherein said first and second portions of the refrigerant blend are vaporized by said indirect heat exchange in said first and second heat exchangers, respectively, and then the first and second portions of the of the refrigerant blend are combined and the resultant combined stream is sent to a first separator D1 where a gaseous fraction and a liquid fraction are separated, and the compression of the refrigerant blend is performed by compressing the gaseous fraction from said first separator D1 in an at least two-stage compressor unit C, a separator D1 upstream of the compressor unit C, a second separator D2 is positioned downstream of the first compressor stage and a third separator D3 is positioned downstream of the second compressor stage, a gaseous fraction removed from the second separator D2 is compressed in the second compressor stage and a liquid fraction removed from the second separator D2 forms said liquid phase, and a gaseous fraction removed from the third separator D3 forms said gas phase, and a liquid fraction removed from the third separator D3 is recycled to said second separator D2.
 18. A method according to claim 1, wherein the hydrocarbon-rich fraction to be liquefied is particular natural gas. 